Ben, welcome to the boards! I can tell you from experience that you are going about this the right way. I've spent about $3000 over the last eight or nine years, upgrading when I found something better, then upgrading again.
I sold my finished costume a couple years ago for less than half what I spent on it, and am starting the next one from the ground up. This time, I know exactly where I'm going to get the best bang for my buck. I hope to be able to do this costume for around $1500, give or take.
These are a great group of folks, and will answer any questions that you have...if you corner them and beat them with a stick. Just kidding. 
Again, welcome!